ABC News(NEW YORK) -- Newt Gingrich lacks the moral character to serve as president, his second ex-wife Marianne told ABC News, saying his campaign positions on the sanctity of marriage and the importance of family values do not square with what she saw during their 18 years of marriage.

In her first television interview since the 1999 divorce, to be broadcast tonight on ABC’s Nightline, Marianne Gingrich, a self-described conservative Republican, said she is coming forward now so voters can know what she knows about her former husband and Speaker of the House.

Watch a preview of ABC News' exclusive broadcast interview with Marianne Gingrich below, and then catch the full interview tonight on ABC News' Nightline at 11:35 p.m. ET.